In a heterojunction field-effect transistor including nitride, as one example of a semiconductor device, when a gate length is shortened to cope with a high frequency, a sectional area of a gate electrode is reduced, and a gate resistance is increased. To above this, there is provided a method in which a metal portion is enlarged in a shape of an opened umbrella (shape of eaves, overhanging and overriding shape) above the gate electrode, the sectional area of the gate electrode is enlarged, and the gate resistance is reduced while a substantial gate length making contact with a semiconductor layer is shortened.
Further, in a heterojunction field-effect transistor made of a semiconductor including nitride, to suppress characteristic current collapse, a dielectric film is inserted between the enlarged metal portion and the semiconductor layer so that an electric filed concentrated on an end of a gate electrode on a side of a drain electrode is alleviated when a high voltage is applied to the drain electrode. A document named as a typical example for this is “Yunju Sun, Lester F. Eastman, “IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. 52”, 2005, p. 1689”.
To shorten the gate length as much as possible and increase the sectional area of the gate electrode in the heterojunction field-effect transistor made of the semiconductor including nitride, it is necessary to provide a structure with the shape of an opened umbrella (shape of eaves, overhanging and overriding shape), and the dielectric film is inserted between the umbrella for increasing the sectional area and the semiconductor. To achieve this, it is necessary to form the dielectric film on the semiconductor before the gate electrode is formed, thereafter, remove, by dry etching, the dielectric film in a region where the gate electrode is formed, and then deposit the gate electrode so that the gate electrode covers the region where the dielectric film is removed.
However, when such a method is used, damage is caused in the semiconductor layer during the dry etching process, and gate leak current and current collapse characteristics deteriorate.
Alternatively, when the dielectric film is formed after the gate electrode is formed, it is difficult to insert the dielectric film in a region between the umbrella and the semiconductor.
When a dielectric film is formed after the gate electrode having a rectangular shape is formed, and further an electrode is formed again so as to cover the gate electrode after the dielectric film immediately above the gate electrode is removed, damaging the semiconductor during the dry etching process can be avoided. However, in this case, it is necessary to perform a lithography process on the gate electrode in resolution higher than that for the gate electrode, and this makes the gate electrode longer for that amount, which causes a problem of not providing a high-frequency characteristic.